Individual semiconductor (integrated circuit) devices (dies) are typically produced by creating several identical devices on a semiconductor wafer, using know techniques of photolithography, deposition, and the like. Generally, these processes are intended to create a plurality of fully-functional integrated circuit devices, prior to singulating (severing) the individual dies from the semiconductor wafer. In practice, however, certain physical defects in the wafer itself and certain defects in the processing of the wafer inevitably lead to some of the dies being "good" (fully-functional) and some of the dies being "bad" (non-functional). It is generally desirable to be able to identify which of the plurality of dies on a wafer are good dies prior to their packaging, and preferably prior to their being singulated from the wafer. To this end, a wafer "tester" or "prober" may advantageously be employed to make a plurality of discrete pressure connections to a like plurality of discrete connection pads (bond pads) on the dies. In this manner, the semiconductor dies can be tested and exercised, prior to singulating the dies from the wafer. A conventional component of a wafer tester is a "probe card" to which a plurality of probe elements are connected--tips of the probe elements effecting the pressure connections to the respective bond pads of the semiconductor dies.
Certain difficulties are inherent in any technique for probing semiconductor dies. For example, modern integrated circuits include many thousands of transistor elements requiring many hundreds of bond pads disposed in close proximity to one another (e.g., 5 mils center-to-center). Moreover, the layout of the bond pads need not be limited to single rows of bond pads disposed close to the peripheral edges of the die (See, e.g., U.S. Pat. No. 5,453,583).
To effect reliable pressure connections between the probe elements and the semiconductor die one must be concerned with several parameters including, but not limited to: alignment, probe force, overdrive, contact force, balanced contact force, scrub, contact resistance, and planarization. A general discussion of these parameters may be found in U.S. Pat. No. 4,837,622, entitled HIGH DENSITY PROBE CARD, incorporated by reference herein, which discloses a high density epoxy ring probe card including a unitary printed circuit board having a central opening adapted to receive a preformed epoxy ring array of probe elements.
Generally, prior art probe card assemblies include a plurality of tungsten needles extending as cantilevers from a surface of a probe card. The tungsten needles may be mounted in any suitable manner to the probe card, such as by the intermediary of an epoxy ring, as discussed hereinabove. Generally, in any case, the needles are wired to terminals of the probe card through the intermediary of a separate and distinct wire connecting the needles to the terminals of the probe card.
Probe cards are typically formed as circular rings, with hundreds of probe elements (needles) extending from an inner periphery of the ring (and wired to terminals of the probe card). Circuit modules, and conductive traces (lines) of preferably equal length, are associated with each of the probe elements. This ring-shape layout makes it difficult, and in some cases impossible, to probe a plurality of unsingulated semiconductor dies (multiple sites) on a wafer, especially when the bond pads of each semiconductor die are arranged in other than two linear arrays along two opposite edges of the semiconductor die.
Wafer testers may alternately employ a probe membrane having a central contact bump area, as is discussed in U.S. Pat. No. 5,422,574, entitled LARGE SCALE PROTRUSION MEMBRANE FOR SEMICONDUCTOR DEVICES UNDER TEST WITH VERY HIGH PIN COUNTS, incorporated by reference herein. As noted in this patent, "A test system typically comprises a test controller for executing and controlling a series of test programs, a wafer dispensing system for mechanically handling and positioning wafers in preparation for testing and a probe card for maintaining an accurate mechanical contact with the device-under-test (DUT)." (column 1, lines 41-46).
Additional references, incorporated by reference herein, as indicative of the state of the art in testing semiconductor devices, include U.S. Pat. Nos. 5,442,282 (TESTING AND EXERCISING INDIVIDUAL UNSINGULATED DIES ON A WAFER); 5,382,898 (HIGH DENSITY PROBE CARD FOR TESTING ELECTRICAL CIRCUITS); 5,378,982 TEST PROBE FOR PANEL HAVING AN OVERLYING PROTECTIVE MEMBER ADJACENT PANEL CONTACTS); 5,339,027 (RIGID-FLEX CIRCUITS WITH RAISED FEATURES AS IC TEST PROBES); 5,180,977 (MEMBRANE PROBE CONTACT BUMP COMPLIANCY SYSTEM); 5,066,907 (PROBE SYSTEM FOR DEVICE AND CIRCUIT TESTING); 4,757,256 (HIGH DENSITY PROBE CARD); 4,161,692 (PROBE DEVICE FOR INTEGRATED CIRCUIT WAFERS); and 3,990,689 (ADJUSTABLE HOLDER ASSEMBLY FOR POSITIONING A VACUUM CHUCK).
Generally, interconnections between electronic components can be classified into the two broad categories of "relatively permanent" and "readily demountable".
An example of a "relatively permanent" connection is a solder joint. Once two components are soldered to one another, a process of unsoldering must be used to separate the components. A wire bond is another example of a "relatively permanent" connection.
An example of a "readily demountable" connection is rigid pins of one electronic component being received by resilient socket elements of another electronic component. The socket elements exert a contact force (pressure) on the pins in an amount sufficient to ensure a reliable electrical connection therebetween.
Interconnection elements intended to make pressure contact with terminals of an electronic component are referred to herein as "springs" or "spring elements". Generally, a certain minimum contact force is desired to effect reliable pressure contact to electronic components (e.g., to terminals on electronic components). For example, a contact (load) force of approximately 15 grams (including as little as 2 grams or less and as much as 150 grams or more, per contact) may be desired to ensure that a reliable electrical connection is made to a terminal of an electronic component which may be contaminated with films on its surface, or which has corrosion or oxidation products on its surface. The minimum contact force required of each spring demands either that the yield strength of the spring material or that the size of the spring element are increased. As a general proposition, the higher the yield strength of a material, the more difficult it will be to work with (e.g., punch, bend, etc.). And the desire to make springs smaller essentially rules out making them larger in cross-section.
Probe elements are a class of spring elements of particular relevance to the present invention. Prior art probe elements are commonly fabricated from tungsten, a relatively hard (high yield strength) material. When it is desired to mount such relatively hard materials to terminals of an electronic component, relatively "hostile" (e.g., high temperature) processes such as brazing are required. Such "hostile" processes are generally not desirable (and often not feasible) in the context of certain relatively "fragile" electronic components such as semiconductor devices. In contrast thereto, wire bonding is an example of a relatively "friendly" processes which is much less potentially damaging to fragile electronic components than brazing. Soldering is another example of a relatively "friendly" process. However, both solder and gold are relatively soft (low yield strength) materials which will not function well as spring elements.
A subtle problem associated with interconnection elements, including spring contacts, is that, often, the terminals of an electronic component are not perfectly coplanar. Interconnection elements lacking in some mechanism incorporated therewith for accommodating these "tolerances" (gross non-planarities) will be hard pressed to make consistent contact pressure contact with the terminals of the electronic component.
The following U.S. Patents, incorporated by reference herein, are cited as being of general interest vis-a-vis making connections, particularly pressure connections, to electronic components: U.S. Pat. Nos. 5,386,344 (FLEX CIRCUIT CARD ELASTOMERIC CABLE CONNECTOR ASSEMBLY); 5,336,380 (SPRING BIASED TAPERED CONTACT ELEMENTS FOR ELECTRICAL CONNECTORS AND INTEGRATED CIRCUIT PACKAGES); 5,317,479 (PLATED COMPLIANT LEAD); 5,086,337 (CONNECTING STRUCTURE OF ELECTRONIC PART AND ELECTRONIC DEVICE USING THE STRUCTURE); 5,067,007 (SEMICONDUCTOR DEVICE HAVING LEADS FOR MOUNTING TO A SURFACE OF A PRINTED CIRCUIT BOARD); 4,989,069 (SEMICONDUCTOR PACKAGE HAVING LEADS THAT BREAK-AWAY FROM SUPPORTS); 4,893,172 (CONNECTING STRUCTURE FOR ELECTRONIC PART AND METHOD OF MANUFACTURING THE SAME); 4,793,814 (ELECTRICAL CIRCUIT BOARD INTERCONNECT); 4,777,564 (LEADFORM FOR USE WITH SURFACE MOUNTED COMPONENTS) ; 4,764,848 (SURFACE MOUNTED ARRAY STRAIN RELIEF DEVICE); 4,667,219 (SEMICONDUCTOR CHIP INTERFACE); 4,642.889 (COMPLIANT INTERCONNECTION AND METHOD THEREFOR); 4,330,165 (PRESS-CONTACT TYPE INTERCONNECTORS); 4,295,700 (INTERCONNECTORS); 4,067,104 (METHOD OF FABRICATING AN ARRAY OF FLEXIBLE METALLIC INTERCONNECTS FOR COUPLING MICROELECTRONICS COMPONENTS); 3,795,037 (ELECTRICAL CONNECTOR DEVICES); 3.616,532 (MULTILAYER PRINTED CIRCUIT ELECTRICAL INTERCONNECTION DEVICE); and 3,509,270 (INTERCONNECTION FOR PRINTED CIRCUITS AND METHOD OF MAKING SAME).